5′-Guanosine monophosphate (hereinafter referred to as “GMP”) is a food additive widely used as a flavor enhancer, like inosine monophosphate (hereinafter referred to as “IMP”). GMP elicits an umami taste and its use is dependent on monosodium glutamate (MSG) also being used. It is often used in synergy with IMP to increase the intensity of the umami taste of MSG.
Examples of the methods for the preparation of GMP known thus far include (1) the enzymatic degradation of yeast RNA, (2) direct microorganism fermentation to GMP, (3) microorganism fermentation to guanosine, followed by chemical phosphorylation, (4) microorganism fermentation to guanosine, followed by enzymatic phosphorylation, (5) microorganism fermentation to xanthosine 5′-monophosphate (hereinafter referred to as “XMP”), followed by conversion into GMP by a corynebacteria strain, and (6) microorganism fermentation to XMP, followed by conversion of XMP into GMP by Escherichia coli which has aminase activity. Of them, method (1) has difficulties of material supply and is economically non-beneficial and method (2) suffers from the disadvantage of being of low yield due to the membrane permeability of GMP. Thus, the other methods are widely used in industrial applications.
For method (6) in which XMP is produced and converted into GMP, it is critical to provide ATP as a cofactor. Most of the ATP used in the conversion of XMP to GMP is supplied from an XMP-producing strain. In the conversion approach, xylene plays an important role because it increases the membrane permeability of ATP and XMP. Xylene in the medium allows ATP and XMP to penetrate into a GMP-producing strain, followed by the conversion of XMP into GMP. Therefore, the approach to GMP production takes the strategy of increasing ATP productivity.
The conversion from XMP into GMP is represented by the following reaction formula:

That is, a continuous supply of ATP, serving as a cofactor, is essential for the GMP-producing process in which XMP is primarily produced by an XMP-producing strain and then converted into GMP in the presence of an enzyme or microorganism having XMP aminase activity. Thus, it is very important to enhance ATP productivity of the XMP-producing strain. The AMP produced in the conversion process is reused as a substrate for ATP production. In fact, adenine-based nucleotides are recycled for the production of ATP.
Hence, increased ATP productivity of the XMP-producing strain is necessary for GMP production in high yield. The activity of malate dehydrogenase has a great influence on the production of ATP. However, nowhere are microorganisms and methods which are designed to enhance malate dehydrogenase activity for increasing GMP production yields mentioned in previous documents.
Keeping in the mind that it is important to increase the ATP productivity of the XMP-producing strain in order to increase GMP yield with priority over the two-step conversion, the present inventors conducted intensive research and found a gene which is responsible for the increase. Also, it was found that when used in combination with XMP animase, a Corynebacteria strain transformed with a recombinant vector carrying two copies of the gene in tandem, could produce XMP and ATP in high yields, thus increasing GMP productivity.